Many biologically active molecules such as hormones, neurotransmitter, etc. transduce their signals by activating PLC through heterotrimeric G proteins. Thus, G protein-mediated activation of PLC plays central roles in a variety of biological processes, ranging from neuronal activities, metabolism, homeostasis, inflammatory responses and some of the sensory processes. Defects in G protein-coupled signal transduction cascades have been linked to many human diseases. Our goals are to investigate the molecular and mechanistic basis of the specificity in interaction between G protein-coupled receptors and G proteins and between O proteins and PLC. Our hypothesis is that specific protein-protein interactions may be one of the major determinants for formation of a specific information processing circuit, which may be responsible for some specific physiological function. Therefore, our study may not only give us insight into the how the G proteins connect the receptors and effectors to form the highly complex signal transduction networks in higher organisms, but also may provide potential targets for developing drugs that enhance or attenuate the biological processes mediated by these signal transduction pathways In this proposal, we will try: 1) To define the receptor sequences involved in coupling to various G-alpha subunits by site-directed mutagenesis and the cotransfection assay system; 2) To further define the sequences of the PLC beta molecules responsible for interaction with G protein alpha and by subunits; 3) To investigate the mechanistic basis of the synergistic effect between G-alpha16 and G-beta-1beta1 on activation of PLC in transfected COS-7 cells.